Lipoatrophy is a highly prevalent complication of antiretroviral (ARV) therapy, associated with decreased quality of life, disincentive for adherence to antiretroviral therapy, as well as possibly an increased risk of coronary artery disease. Lipoatrophy has been shown to correlate with mitochondrial abnormalities, mainly with histological abnormalities and decreased subcutaneous fat mtDNA content, but the exact mechanism of this dysfunction and its importance in the pathogenesis of lipoatrophy remain unclear. Recent work from our group suggested an association between oxidative stress and lipoatrophy. Reactive oxygen species (ROS) contribute to oxidative stress and are produced principally by the mitochondria during oxidative phosphorylation (OXPHOS). ROS damages mitochondrial DNA (mtDNA), lipids, and proteins leading to a decrease in expression of mtDNA related products involved in OXPHOS, and subsequent decrease in ATP synthesis. Whether the finding of increased oxidative stress represents the consequence of or the cause of this mitochondrial dysfunction is unclear. Our long-range goal is to understand the role played by mitochondria in the development and reversal of lipoatrophy. We hypothesize that the initiation of nucleoside analogue reverse transcriptase inhibitor (NRTI)-containing therapy leads to alteration in mtDNA synthesis which will be assessed by a decrease in mtDNA levels and a resultant decrease in synthesis of mtDNA-encoded respiratory chain subunits with ultimately a defect in OXPHOS. The latter will in turn lead to an increase in fat apoptosis and in oxidation of mitochondria and lipids. This will be assessed in longitudinal studies of subjects who are starting their first NRTI-containing antiretroviral regimen. In addition, we propose to study 2 types of interventions to reduce lipoatrophy and facilitate our understanding of the role of mitochondrial recovery in the regression of lipoatrophy: (1) decreasing exposure to inducers of lipoatrophy by substitution of the mitochondrial friendly NRTI tenofovir, for d4T, and (2) use of uridine supplementation. We hypothesize that both interventions will be able to replete mtDNA content in tissues, followed by improvement of mitochondrial function and decrease fat apoptosis and oxidative damage, with subsequent increase in subcutaneous fat. Blood, skeletal muscle, and subcutaneous fat biopsies will be obtained from patients prior to, and 24, and 48 weeks after the clinical intervention, to measure changes in mtDNA content, mitochondrial functional assays, oxidative damage biomarkers, and fat apoptosis.